What are the main minerals of iron ore?

The main ore minerals of iron are the oxides hematite (Fe2O3) and magnetite (Fe3O4) and the hydroxide goethite (FeO.OH). Important in some deposits are mixtures of hydrous iron oxides that go by the blanket name limonite and have no fixed formula.

Hematite (in the old spelling haematite), gets its name from the blood red streak it gives when it is rubbed across a rough surface. It is a dense, hard, non magnetic mineral that occurs in many different forms, two of the commonest being crystalline masses and as an earthy rubble. An important form in many of the world’s largest and richest deposits is made up of a network of hematite platelets, most of them fractions of a millimetre across. A related form consisting of tiny, glittering scales was being mined underground as much as 45,000 years ago on Bomvu Ridge in Swaziland (though probably for decorative purposes rather than as iron ore).

Magnetite is a black, highly magnetic mineral that resembles hematite in hardness and specific gravity but gives a black streak rather than a red one. It is equally widespread in nature, and the magnetic properties of rocks depend to a large extent on their content of magnetite. It was by using chunks of magnetite (lodestones) to magnetize iron filings that the Chinese were able to make primitive compasses as long ago as the seventh century AD. Magnetite is easy to concentrate using an artificial magnetic field, and the iron content of the concentrates can come close to the theoretical maximum of 72.4 per cent.